1. Field of the Disclosure
Embodiments disclosed herein relate generally to an apparatus for the injection of a gaseous stream into a bed of fluidized solids. More specifically, embodiments disclosed herein relate to an injection nozzle.
2. Background
In the refining and chemical process industries, as well as in other processing industries, it is often necessary to inject a gaseous stream into a bed of finely divided solids, uniformly spreading the gas over the cross section of the bed and. The injection of the gas is designed to promote uniform and intimate contact of the gaseous medium with the bed of solids so as to achieve a purpose, such as a chemical reaction between the gas and solids and/or a mass transfer operation between the gas and solids.
Apparatus for injecting the gas into a fluidized bed typically consists of a flat grid plate with holes, a pipe grid system, or a series of concentric rings. These distributors are designed to physically cover as much of the bed cross section as possible so as to promote the even distribution of the gas across the entire bed. Gas is introduced into the space beneath the flat plate distributor or into the main header of a pipe grid and/or the ring distributor from a central source. From there the gas flows throughout the pipe grid or ring system and then discharges into the bed through a multiplicity of nozzles that distribute the gas uniformly into the bed. Plate grid distributors are typically not completely flat but are dished slightly up or down so as to better withstand the pressure exerted by the gas and/or the weight of the bed of solids above. Plate grid distributors may or may not contain nozzles, but typically only use a pattern of holes laid out in the plate to allow gas to flow through into the bed. Other embodiments of gas distributors for fluid beds of solids include dome type distributors and so-called “mushroom” distributors.
In order to achieve uniform distribution of the gas medium, the injection nozzles are typically designed with a cross sectional area that will cause a pressure drop to occur as the gas flows through the injection nozzles from the distribution header into the bed of solids. The maintenance of a positive pressure drop across the injection nozzles insures that the gas flows evenly to all of the injection nozzles in spite of differences that can occur in the pressure in the bed at the point of discharge. Once the gas flows upward through the bed of solids, the bed becomes “fluidized” and begins to behave as a liquid. Such a fluidized bed of solids will exert a pressure proportional to the depth of the bed and the density of gas/solids mixture in the same manner as would a liquid of similar density and depth. Typically, such beds of fluidized solids will range in depth from a few feet to as much as 30 feet or more and will exhibit a measured density ranging from a few pounds per cubic foot to over 40 pounds per cubic foot. The resulting pressure exerted by the column of fluidized solids will range from as little as 1 pound per square inch (psi) to as high as 10 psi or more. Moreover, the bed of solids is often quite turbulent, meaning the pressure at any one point in the bed fluctuates with time and will vary from point to point at a given depth in the bed. For this reason, it is important to design gas distributors with sufficient pressure drop so as to overcome the pressure fluctuations that are expected at the location of the gas distributor in the bed. A typical “rule of thumb” for the design of gas distributors is that the minimum pressure drop should be 15% of the bed pressure drop for downward pointed injection nozzles and 30% of the bed pressure drop for upward pointed distributors.
In addition to maintaining a minimum pressure drop for uniform distribution of the gas medium, injection nozzles are also designed to discharge the gas into the bed at relatively high velocity. If the gas velocity is too low, pressure pulsations can momentarily cause solids to be pressured backwards and flow from the bed into the injection nozzle. Such backflow of solids into an injection nozzle is undesirable as it can lead to erosion of the injection nozzle from the continued scouring action of the solids and/or plugging of an injection nozzle if the solids become lodged into a solid mass. Moreover, if the solids are pressured far enough into an injection nozzle, they can then enter the main header where they are picked up by the flowing gas to be discharged in one or more injection nozzles farther downstream. In such latter case, the result may be severe erosion in one or more injection nozzles downstream from the nozzle through which the solids entered the header. To prevent backflow of solids into the injection nozzles, the velocity in the nozzles is generally maintained above a certain minimum value, typically above about 20 feet per second (fps).
A problem that continues to plague gas distributors in fluid beds is erosion of the injector nozzles at the point of discharge into the bed of solids. Over a long period of time, the impact of solid particles at the discharge edge of the injector will cause gradual wear at the injector tip. As the wear increases, the end of the nozzle can erode far enough back so as to destroy the point of attachment where the injector nozzle passes through the header. The result is a hole in the header and a loss of performance of the distributor. When this occurs, expensive and time consuming repairs are required to restore the performance of the grid or ring.
One widely used process in the petroleum refining industry that makes use of beds of finely divided solids is the fluid catalytic cracking (FCC) process. The FCC process is used for the cracking of heavy boiling gas oil streams to produce more valuable, lighter boiling products, such as gasoline and lighter hydrocarbons. The FCC process uses solid catalysts in powder form to facilitate the breaking of the carbon-carbon atomic bonds of the gas oil feed to form smaller molecules that lie within the gasoline boiling range. In addition to the gasoline product, the process also produces substantial yields of lighter gases, such as propane and butane, which are recovered and converted to valuable products. Fluid catalytic cracking is the most widely used “conversion” process in petroleum refining and several million barrels per day of FCC capacity have been installed since the process inception in the early 1940's. As such, the FCC process is of great economic value and is typically the most profitable unit in a petroleum refinery in the United States as well as in most refineries around the world.
The catalyst used in the FCC process is a finely divided solid composed of mostly silica and alumina in both crystalline and amorphous form. The use of a powdered catalyst has been the key feature contributing to the success of the FCC process and has lead to the development of an entire area of process operations that has come to be known as “fluidization.” The finely divided powder catalyst can be made to behave as a fluid when it is properly aerated or “fluidized” by means of air or another gas. The fluidized powder can be made to flow in lines and will establish a level within a vessel, as would a liquid. A fluidized power will also generate a hydraulic pressure head proportional to the density and the depth of the mixture within a vessel or in a vertical standpipe as would a fluid. The powder can also be pneumatically transported by a gas stream when the gas has sufficient velocity. The ability to flow the powdered catalyst between vessels has been of tremendous benefit in the development of a viable catalytic cracking process. Earlier attempts to use a fixed bed of catalyst pellets were largely handicapped by the need to regenerate the catalyst frequently to remove deposits of “coke” that are a by-product of cracking. The coke, mostly carbon with some hydrogen and sulfur, deactivates the catalyst and must be removed by means of a combustion step. By use of a “fluidizable” catalyst, the catalyst can be continuously circulated between the reaction and regeneration vessels of a FCC unit so that there is no need for a cyclical process in order to accomplish the reaction and regeneration steps.
In the FCC process large volumes of air are used in the Regenerator vessel to remove coke from the catalyst and restore catalyst activity. The air is typically injected into a bed of coked catalyst by means of pipe grid or ring type distributors. The FCC process also makes use of large pipe or ring type distributors in the stripping section where the spent catalyst is contacted with steam to remove entrained hydrocarbons before the catalyst is sent to the regenerator. Smaller pipe or ring distributors are used elsewhere in the FCC process to inject either steam or air to keep the powdered catalyst in a “fluidized” or aerated state. Injection nozzles used in each of these areas of the FCC process may be subject to erosion, as described above.
A number of attempts have been made to improve injection nozzle design to reduce the harmful effects of erosion and extend the useful life of the distributor. These include the use of exotic alloys and ceramic materials to make the injection nozzle itself harder and more resistant to erosion, protecting the discharge end of the injection nozzle with hard surfacing such as metal overlays or refractory layers, and/or changes in the design of the nozzle.
A current state-of-the-art air distributor design for an FCC regenerator makes use of a pipe grid distributor and a two-stage injection nozzle design. Such a design is shown in FIG. 1A, which is a plan view of a pipe grid distributor consisting of three identical grids in a triangular arrangement to cover the circular cross-section of an FCC Regenerator. FIG. 1B is an elevation view of one of the pipe grids showing how the main air supply trunk enters from the bottom, with three branches that also support the grid. All of the grids are installed at the same elevation in the bed of powdered catalyst so as to have, as nearly as possible, a uniform pressure into which the air is discharged. FIG. 1C is a cross section of one of the grid arms showing the arrangement of the downward-directed nozzles at 45 degrees from vertical. This pipe grid design has proven very efficient at physically distributing the air over the cross section of the catalyst bed so that the coke burning is uniform.
FIG. 1D is a cross section of one of the air injection nozzles that utilizes a two-stage or so-called “Borda” tube. FIGS. 1E-1H contains various views showing the nozzle arrangement inside one of the grid arms. The metal grid arms are covered in erosion resistant refractory to protect both the grid arms themselves and the final length of the injection nozzles from erosion by the wearing action of the catalyst particles.
The Borda tube or two-stage nozzle consists of a straight tube with a concentric orifice at the inlet end. In the Borda tube design, the orifice is sized so as to provide sufficient pressure drop to promote uniform distribution of air across the grid, where the pressure drop is typically between about 1 and 3 psi. The orifice is followed by a larger diameter tubular section that slows down the gas so that the discharge velocity into the bed of solids does not cause excessive erosion and/or attrition of the catalyst. See, for example, Joseph W. Wilson, “Fluid Catalytic Cracking,” p. 140-141, Penwell Publishing, 1997, describing use of a Borda tube as an injection nozzle in FCC applications.
The recommended length for the Borda tube is a minimum of six times the tube diameter to allow the flow in the tube to become fully developed following the nozzle orifice. It has been established in practice that, if the nozzle is too short, the flow at the discharge will be turbulent and excessive erosion will result at the nozzle tip.
Although the Borda tube design and the use of hard materials for construction of the nozzle and/or protection of the nozzle tip have greatly improved the life of the grid distributors in FCC service, these designs are still subject to erosive wear that requires periodic replacement of either individual nozzles in a grid arm or replacement of the entire grid arm. These types of repairs are difficult to make and can lengthen the time required to perform routine maintenance during scheduled down periods.
Another example of an air distributor used for FCC catalyst regeneration process is disclosed in U.S. Pat. No. 4,223,843. As disclosed therein, the air distributor includes a plurality of nozzles in a header ring and in a cylindrical housing, with each nozzle formed with a diverging bore for ejecting high pressure air. The diverging bore of each of the nozzles is formed at a half angle of less than 7° for providing a maximum air velocity without destruction of the spent catalyst.
U.S. Pat. No. 4,460,130 discloses an injector nozzle disposed external to the manifold having a central opening and an inlet extending from the nozzle to the central passage. The cross-sectional area of the central opening in the nozzle in the direction of flow is smaller at least at one point than that of the inlet such that a major portion of the pressure drop in the gas flowing from the central passage through the inlet and the nozzle is created by the nozzle. The central opening outwardly diverges at an angle of less than 15° to avoid jetting and formation of eddy currents.
There still exists a need in the art for gas distribution devices that are subject to less erosive wear.